Polymer & Co.
nGen_Flex
What is ngen_flex ?
nGen_Flex is a semiflexible filament manufactured by colorFabb using thermoplastic copolyester elastomers (TPC-E), a class of polymers that presents the flexibility of rubbers and can be easily remodeled using heat, as thermoplastics. In addition, these materials present high solvent and heat resistance (which is translated into a high chemical resistance and thermal stability in our project).
Physico-chemical properties
The main features of nGen_Flex:
1. Temperature Resistance: it is made to withstand temperatures up to 130ºC.
2. Flexibility and Toughness: it can be used to print firm and tough objects or semiflexible ones according to the polymer filling percentage chosen.
3. Fast Printing: different from other flexible thermoplastic elastomers used in 3D printing, nGen_Flex is semiflexible, which means that it can withstand higher pressures in the extruder thus higher printing speeds.
Thermoplastic elastomers
Thermoplastic elastomers, combine physical properties of vulcanized rubbers (high flexibility) with the easy processing properties of thermoplastics (which can be easily modeled after polymerization at high enough temperatures). They present a biphasic system, which is composed of a hard and solid (crystalline) phase, providing strength, and a soft (rubbery) phase, providing flexibility and impact resistance (figure 1).
These polymers are processed at temperatures high enough to melt the crystalline phase, allowing flow. During cooling, this phase solidifies and acts as a link between the polymer chains of soft phase (physical crosslinks), this link is a constraint to molecular movement and provide the elastic behaviour typical of rubbers (figure 2). [3]
In thermosets, on the other hand, the crosslinks are made of covalent bonds, normally created during the vulcanisation with sulphur molecules. Once the reaction is completed, there is no way to remodel thermosets.
Thermoplastic Copolyester Elastomers
TPC-Es present a broad range of operation temperatures, typically, having a higher Tg up to 70ºC, a lower Tg around -52ºC and a high melting temperature above 200ºC (which is not the case for nGen_Flex, as seen in table 1). They also show very good resistance to oxidation at high temperatures and good chemical stability, especially against apolar solvents. Moreover, they present excellent fatigue resistance. [4]
The commercial copolyester elastomers are mostly based on poly(tetramethyleneoxide) (PTMO) as flexible segment and poly(butyleneterephthalate) (PBT) as the rigid segment. They are synthesised by a step-growth polymerization process of dimethyl terephthalate (DMT) with a short chain diol (here BDO) and a long chain one (PTMO), as presented in figure 3. [4]
Figure 3: Example of Polymerization Mechanism to obtain a TPC-E
3D-Printing test
In the following table, one can find the theoretical properties describing the material provided by the manufacturer [2] :
Experimental analysis
3D-Printing test
To determine nGen_Flex printing conditions that best match our printer specifications (Makerbot Replicator 2X), a dogbone model was printed. Several trials were done until we arrived at the following critical conditions:
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Printing temperature: ~250ºC
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Bed Temperature: 110ºC
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Bed surface: Blue Tape
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Raft: no
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Support: no
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Printing Speed: 50-90 mm/s
Figure 4: Dogbone in nGen_flex
DSC
From the DSC analysis, it is possible to see that nGen_Flex presents a crystallization at TC≅165ºC during cooling, which seems to achieve completion in the second heating cycle at T≅83ºC, and melts atTm≅210ºC, with subsequent degradation events above T=240ºC, whose nature remains to be understood.
As expected for thermoplastic elastomers, this polymer presents a crystalline phase (evidenced by the melting and recrystallization events) and an amorphous one (with Tg below room temperature around -10ºC, as presented by the DMA analysis).
Figure 5: DSC nGen_Flex (10ºC/min) from 35ºC to 265ºC
Rheometry
The rheometric study of nGen_Flex was performed under 3 temperatures, 200ºC, 240ºC and 260ºC. Those temperatures were chosen to be respectively 30ºC above the Vicat A Softening Temperature (one of the possible definitions of softening point for materials that have no definite melting point, such as plastics), and in the extremes of the printing temperatures. The deformation regime was linear (constant strain below 30% for frequencies between 0.1 and 100 rad/s).
Figure 6: Disk-shaped sample
Figure 7: Time Sweep at 200º showing the increase of G' and G'' over time
This sample has proven not to be thermorheologically simple. The results present a structure evolving over time, as presented by the Time Sweep, a test mode in which the temperature, frequency and strain constant and evaluate the evolution of the modulus over the time
For 200ºC and 240ºC it becomes more solid-like, presenting crossover points, crosses in Figures 8 and 9, for higher frequencies and higher modulus as the time passed. The crossover point is observed when G' exceeds G" or the Loss Factor (G”/G’) becomes less than the unit, marking the transition to the rubbery region where there is solid-like behavior (elastic dominant).
Figure 8: Frequency Sweep at 200ºC (crosses present the crossover points)
Figure 9: Frequency Sweep at 240ºC (crosses present the crossover points)
This trend becomes clearer when analyzing the Loss Factor.
Figure 10: Loss Factor calculated for the test performed at 200ºC
Figure 11: Loss Factor calculated for the test performed at 240ºC
The exact reason for this evolution remains to be understood in further analysis. Once the present sample is composed of a polyester, it is very likely that this behavior results from a chain association caused by transesterification.
On the other hand, the test performed at 260ºC presented a different evolution of the Loss Factor. Sample 3 became more liquid-like than the two previous for longer times, which can be seen in Figure 13. This behavior could be related to the degradation peaks previously seen in the DSC analysis.
Figure 12: Frequency Sweep at 260ºC (crosses present the crossover points)
Figure 13: Loss Factor calculated for the test performed at 260ºC
COnclusion
By João
Although nGen_Flex has proven to be a difficult material to study, presenting a non-thermorheologically simple behavior, data obtained from DSC and Rheometric analysis can assure its thermal resistance below 120ºC (our operating temperatures). The time evolution seen during the rheometric study should not be a cause for concern once the filament does not remain at those temperatures for a long period of time during printing.
However, nGen_Flex is not difficult to use. This filament can be printed relatively fast and without major problems if the correct setup is applied.
Moreover, because of its rubber-like properties, nGen_Flex is very desirable for parts in which adhesion is needed, such as the clamp surface.
These are the main reasons that led us to choose nGen_Flex as one of the polymers used to print our model.
Bibliography
[1] color Fabb, “color Fabb nGen_Flex”, Consulted: 29/04/2018, Retrieved from: https://ngen-flex.colorfabb.com
[2] Eastman lab, “ Technical Data Sheet Eastman Amphora™ Flex 3D Polymer FL6000”, 12-Sep-2016 7:30:52 AM, Retrieved from: http://ws.eastman.com/ProductCatalogApps/PageControllers/ProdDatasheet_PC.aspx?product=71107028
[3] Wikimedia Commons, “Polymer_picture.svg”, Consulted: 29/04/2018, Retrieved from: https://en.wikipedia.org/’wiki/Elastomer#/media/File:Polymer_picture.svg
[4] Turkish Plastics Industry Foundation, “Thermoplastic polyester elastomer (TPE E)”, Consulted: 29/04/2018, Retrieved from: https://www.pagev.org/tpe-e-en
[5] Robert Shanks and Ing Kong, “Thermoplastic Elastomers”, Applied Sciences, RMIT University, Melbourne Australia, Chapter 8
[6] © Intertek Group plc , "Vicat Softening Temperature ASTM D1525, ISO 30", Consulted: 07/05/2018, Retrieved from: http://www.intertek.com/polymers/testlopedia/vicat-softening-temperature-astm-d1525/